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After getting my Optec TCF-S focuser, I realized that one thing that prevented me from focusing quickly was never knowing exactly where my primary mirror is. Since it is geared to the focus knob, I decided that if I could monitor the rotations of the knob, I would know exactly where the mirror is located. This would allow me to get near to focus quickly.

This works great. After caliibrating everything on a cloudy night, I can now get to focus on any combination of equipment that I use in seconds and just fine tune it with the actual CCD images. Rotating the focus knob updates the counter about 34 counts, so each count means the focuser has rotated just over 10 degrees.

I found some misc. parts that seemed to fit the requirements - a counter with 10 counts per revolution, a miniature stainless steel chain, and a small sprocket for the counter, but I couldn't find anything that would fit over the focus knob. I ended up making a sprocket myself out of nylon with a small lathe and grinder.

As you can see in the first photo, I left the base wide enough to hold a setscrew to make sure it doesn't slip.


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